The macrophage, an important component of the immune system, in large part functions to detect and damage foreign cells. The interaction between human macrophages and human erythrocyte target cells serves as a study model for immune macrophage-lymphoblastoid cell interaction. Human erythrocytes coated with IgG lymphoblastoid cells or C3 have been observed to bind to homologous macrophages. Macrophage-lymphoblastoid cell interaction is being examined as a model for macrophage tumor cell interaction. A spontaneous interaction has been observed and characterized. The capacity of complement to augment this interaction is being studied, using tissue as well as circulating macrophages, as well as the effect of the pharmacologic agents, corticosteroids, levamisole, and BCG. Delineation of the mechanisms involved in macrophage recognition and recognition of altered cell surfaces should provide insight into new approaches for harnessing the macrophage's ability to detect tumor cells and modify their viability. The role of macrophage subsets is also being examined as well as the role of the macrophage in the regulation of normal and disordered immunoglobulin production.